Continuously increasing demand for higher productivity in machining difficult-to-cut materials is driving the development of high-performance cutting tool materials and cutting approaches. To achieve high machining productivity, a marked trend in cutting tools is to increase cutting speed. However, for cutting speeds in excess of 1500 sfm (surface feet per minute), the cemented carbide tools which are widely used now will lose their strength and the tool nose will be deformed due to the high temperature (usually over 800° C.) introduced by high speed cutting. These will also affect the dimensional tolerance of the workpiece and lead to a shorter tool life. Coolant is usually used in machining to reduce the temperature at the cutting edge and thus extend the tool life. However, to the concern in cost saving and environmental protection, dry cutting operations are always required.
With high resistance to heat and wear, superior hot hardness and chemical stability, ceramic cutting tools can be used to machine metals that are extremely hard and abrasive. These unique properties of ceramics can also allow them to be used to machine metals at much higher cutting speeds than the carbide tools even in dry machining conditions. The most commonly used ceramic materials for cutting tools are alumina and silicon nitride. Alumina based materials are excellent candidates for cutting tools because of their good chemical stability and abrasion resistance at high temperature. However, intrinsic brittleness, low strength and low thermal shock resistance of the alumina based ceramic cutting tools limit their applications. Silicon nitride based cutting tools, which have good wear resistance, high hardness, excellent high-temperature properties, and most importantly, better fracture toughness than the alumina based cutting tools, are widely used in machining metals, more particularly for machining cast iron at high speeds. Typically, the fracture toughness of Si3N4 ceramic can be improved by adding elongated β-Si3N4 particles into α-Si3N4 materials. The increased fracture toughness is attributed to the energy dissipation during crack propagation via crack deflection and elastic bridging by the β-Si3N4 phase.
The Si—N bonds in Si3N4 materials have mostly the covalent character and, the solid-state diffusions are very slow, which prevents the densification of silicon nitride at high temperature. An efficient method of producing pore-less silicon nitride ceramic materials consists of introducing active additives during the sintering process and using pressure enhanced densification techniques, such as hot pressing (HP) or isostatic hot pressing (HIP). At the sintering temperatures, the densification facilitating substances, usually oxides (such as Al2O3, Y2O3, MgO, CeO2, ZrO, BeO and La2O3), react with the surfaces of the Si3N4, forming a liquid phase, which makes possible mass transport processes and enhances the densification.
Ceramic materials such as silicon nitride have a very high electrical resistivity (1×1012·Ω·cm), the electrical property of Si3N4 materials can be tuned by the addition of electro-conductive phases. Recently, there is a growing interest in synthesizing electrically conductive silicon nitride composites, while improving the mechanical properties of the sintered articles. In this context, conductive additives such as TiN, SiC, etc., are added into silicon nitride to improve its electrical conductivity. It has been reported that Si3N4—TiN, Si3N4—TiC, Si3N4—MoSi2, Si3N4—SiC, Si3N4—TiCN, Si3N4—TiB and Si3N4—MoSi2—SiC composites have been successfully synthesized. It was also reported that silicon nitride ceramic with Na2O—Al2O3—SiO2 glass as the grain boundary phases resulted in two and four orders of magnitude higher electrical conductivity than that of Si3N4 ceramic with Y2O3 and Al2O3 additives at 100 and 1000° C., respectively. One advantage is that such composites can be shaped into complex geometries by the more economical electrical discharge machining (EDM). The other advantage is that the strength and toughness of the sintered particles can be much improved by the addition of second-phase particles by mechanisms including residual stresses generated by the mismatch of coefficients of thermal expansion (CTE), crack bridging and crack deflection.
Moreover, ceramics based on silicon nitride offer increased resistance to abrasion and thermal shock and have high fracture toughness. Along with the new material development based on high purity ceramic powders, optimized powder processing and sintering techniques, the new silicon nitride grade is characterized by reduced content of grain boundary phase and an extremely homogeneous fine microstructure. This results in a higher bending strength and wear resistance combined with high fracture toughness especially at very high temperatures. Nowadays, Si3N4-based cutting tools have been widely used in high-speed machining cast alloys, hardened steels and high-Ni superalloys, especially on rough finished turning and milling, where super toughness and wear resistance are required. The Si3N4-based cutting tools can be used to machine cast irons, hardened steels and Ni-based superalloys with surface speeds up to 25 times higher than those obtained with conventional materials such as tungsten carbide. Furthermore, Si3N4 based ceramics have been found to be an excellent choice for machining grey and ductile cast irons at cutting speed over 400 m/min.
There was a predominant opinion that coating on ceramic cutting tools and inserts was meaningless, because of the high hot hardness and chemical inertness of ceramics. This opinion has been changed recently, when beneficial influence of coatings on functional properties of ceramic tools and inserts was found. On one hand, the influence of coating on increasing the tool life is interpreted by a decrease in heat emission during machining, due to a decrease in the coefficient of friction between the cutting tool and the workpiece and also due to a lower probability of chipping, by eliminating their initiation sites on the inserts. On the other hand, when the cutting speeds exceed 1500 surface feet per minute (sfm), the temperature at contact faces of the cutting inserts may be over 1000° C. due to the heat generated by dramatic friction between the cutting tools and the workpiece. Under this cutting condition, the chemical inertness of cutting tool material becomes more and more important. However, at this temperature, silicon nitride-based ceramic cutting tools exhibit lower chemical degradation than desired in machining iron-based alloys, such as cast iron and steels, due to the chemical reaction between silicon nitride and iron. Serious diffusion-related crater wear can be usually observed in this application. This crater wear is believed resulting from the chemical affinity between Si3N4 elements and iron alloy components that promotes tribo-chemical reactions. During high speed machining, oxidation wear occurs locally which causes notch wear at cutting edges. Therefore, chemical inertness of the cutting tool material becomes more important under high-speed, dry machining conditions.
Several ways have been developed for improving the chemical stability of the silicon nitride-based ceramic cutting tools. By addition of inert metal oxide (SiO2, Al2O3, etc.) as the second phase to the Si3N4 matrix is one way to improve its chemical inertness and therefore the tool life. It is reported that wear resistance of Si3N4 tool for machining iron increased with Al2O3 content. However, higher Al2O3 content also induces higher brittleness and thus reduces the performance of silicon nitride-based ceramic cutting tools.
A more effective way to solve this problem is to apply protective coating(s) on the surface of the Si3N4 based ceramic cutting tools. The coating(s) can form an excellent thermal and chemical barrier between the tool and the workpiece, and meanwhile, can keep the excellent physical and mechanical properties of the Si3N4 ceramic materials. Therefore, the cutting performance and the tool life can be significantly improved.
Various coatings have been successfully applied on cutting tools and inserts where the tool materials are high speed steels or cemented carbide, from single, binary coatings to complex multi-component and multi-layer configurations. Among them, the most commonly used coating materials for cutting tools over the past years are transition metal nitrides, such as TiN and TiAlN. TiN coating is good for protecting cutting tools due to its high hardness and wear resistance. However, the application of TiN coating is limited to low temperatures since it tends to be oxidized at temperatures above 600° C. The oxidation resistance of TiAlN coating is much higher than that of TiN with the formation of a dense and strongly adhesive Al2O3 by Al atoms diffusing to the surface. Although the oxidation temperature of TiAlN coating can be improved to 800° C., it is still not good enough for high-speed cutting with temperatures over 1000° C.
Al2O3 has excellent wear protection, high hot hardness and stability at elevated temperatures above 1000° C., at which most nitride coatings suffer from severed and rapid oxidation. Having such various advantages, Al2O3 is a good candidate material for protective coating(s) on cutting tools to improve their productivity of machining operations, especially at high temperature under high-speed and dry machining conditions.
The most commonly used methods for producing coating layers on cutting tools are chemical vapor deposition (CVD) and physical vapor deposition (PVD). So far, due to the requirement for high deposition temperature and insulating attribute of Al2O3, the mostly used technique for producing Al2O3 and their multi-layered coatings is chemical vapor deposition (CVD). For Si3N4 cutting tools which are also insulating, the main technique to produce Al2O3 coating(s) is also CVD method.
CVD methods have been proven to be excellent processes for hard coatings and offer advantages of uniform coating even for workpiece of complicated geometries. In the past, refractory coatings, such as TiC, TiCN, TiN, and Al2O3, or their combinations, have been applied by CVD techniques. The CVD coatings on cutting tools have been developed from TiC, TiCN, TiN, and Al2O3 coatings to complex TiC/TiC/TiCN, TiC/Al2O3/TiN, TiC/TiCN/Al2O3/TiN coatings which are mostly the combination of above mentioned coating materials. In those coatings, each layer provides an attractive feature that gives superior coating properties. TiN, TiC and TiCN can increase the hardness and reduce friction from the contact forces while the Al2O3 layer provides thermal and chemical barrier under elevated temperatures.
Al2O3 crystallizes into several different phases such as α, κ, and χ called the “α-series” with hcp stacking of the oxygen atoms, and into γ, θ, η and δ called the “γ-series” with fcc stacking of the oxygen atoms. The CVD processes are normally operated at temperatures in excess of 1000° C., at which α- and κ-Al2O3 may be synthesized with grain size in the range of 0.5-5 μm and having well-facetted grain structures. However, due to the high deposition temperatures, coatings produced by CVD technique are characterized by thermal cracks, and residual tensile stresses caused by the thermal expansion mismatch between the coating and the substrate as the tool is cooled down from the processing temperature to the room temperature. The tensile stress may exceed the rupture limit of the Al2O3 to create cracks on coating. During machining, the coating may crack due to the reduction in the transverse rupture strength (TRS) of the insert which produces a greater susceptibility to chipping and breakage.
A lot of other drawbacks related to CVD process, such as coarse grain structure, explosive and corrosive reactive gas that induces dangers and corrosion of the coating chamber, environmental pollutions, limit the application of the CVD coatings. These drawbacks can be avoided in deposition processes at lower substrate temperatures for the synthesis of crystalline films, like pulsed DC (Direct current) plasma-assisted CVD, medium temperature CVD or PVD techniques. MTCVD (medium-temperature CVD) coatings, as the name implies, are deposited at lower temperatures than CVD coatings which eliminate cracks formation in the coating. As a result, MTCVD coatings offer advantages of increased toughness and smoothness without sacrificing wear resistance or crater resistance. Tools with these coatings covered have broader application ranges for ferrous materials, allowing consumers to inventory fewer grades and, therefore, suffer fewer application mistakes. By the development of plasma assisted CVD technique, coatings can also be deposited at lower temperature. At temperatures about 450-700° C., the dominance of the thermal stress can be avoided and thus the thermal distortion is reduced.
The introduction of PVD coatings for cutting tools in the metal cutting industry is one of the main success stories in the industrial application of modern coating technology over the last 30 years. The first PVD coating material for commercial application on cutting tools was TiN in the early 1980s. Since the 1990s most cutting tools have been PVD coated particularly in applications where sharp edges are required, e.g. threading, grooving, end-milling, etc. and in cutting applications that have a high demand for a tough cutting edge, e.g. drilling.
PVD methods tend to develop a residual compressive stress caused mainly due to the non-equilibrium plasma deposition environment in the coating. The compressive stress can delay the onset of thermal cracking if the coating can withstand the heat and remain adherent at the cutting edge of the tool. Once the onset of thermal cracking is delayed, tool life can be significantly enhanced during the machining operation. Other merits of the PVD coatings compared with CVD coatings include: relatively low deposition temperatures; preserving cutting edge toughness; being able to be applied to sharp cutting edges; finer grains (smoother) with higher micro-hardness; non-equilibrium compositions impossible with CVD.
Several PVD techniques have been developed in the past several years. For the application of coatings on cutting tools, the mainly used techniques are: cathodic arc deposition (or called arc ion plating), sputter deposition (or called magnetron sputtering), pulsed laser deposition and electron beam physical vapor deposition.
In cathodic arc deposition, a high power electric arc discharged at the target (source) material blasts away some into highly ionized vapor to be deposited onto the substrates. The principal advantage of this arc discharge is the high degree of ionization, which increases the adhesion of the film to the substrate and improves crystal growth. The disadvantage of this method is that macro-particles from cathodic spots result in voids in the films.
Sputter deposition is the deposition of particles vaporized from the target by the physical sputtering process which is a non-thermal vaporization process where the atoms are ejected from the surface of a glow plasma discharge (usually localized around the “target” by a magnet) bombards the material sputtering some away as a vapor for subsequent deposition. Uniform thin films can be obtained easily by this method, and the strong ion bombardment which is inevitable in the cathodic arc evaporation can be avoided.
In pulsed laser deposition (PLD) a high power laser ablates material from the target into a vapor and, hence, forming dense plasma inside the chamber of the evaporated material. PLD is a useful technique to produce high quality multilayer films which are continuous and smooth without using substrate heating.
Electron beam physical vapor deposition (EBPVD) is an important coating method in which the material to be deposited is heated to a high vapor pressure by electron bombardment in “high” vacuum and is transported by diffusion to be deposited by condensation on the (cooler) workpiece.
Each method mentioned above has its own merits. Coatings produced by different methods have different microstructures, grain size, and hardness, state of stress, intrinsic cohesion and adhesion to the underlying substrate bodies. The mostly utilized methods for depositing coatings on cutting tools are cathodic arc deposition and sputtering deposition. As each technique has its own limitation, many new techniques based on the cathodic arc deposition and sputtering deposition had been developed by utilizing different magnetron fields and different cathode powers. For example, the filtered arc and nano dispersed arc jet by utilizing an electromagnetic filter or a special designed magnetic field enables solution of the main drawback of macro-particle pollution in arc technology. The closed unbalanced magnetic field in magnetron sputtering, application of high power pulsed power on the magnetron cathode improves ionization in sputtering, thus leading to formation of high-quality dense films. The application of mid-frequency pulsed power and radio-frequency power on the magnetron cathode enables the deposition of not so conductive or insulating materials.
As a consequence of recent developments in PVD technology, many new coatings have been developed. Coatings made of oxide, nitride and super-hard materials have come to the marketplace. The coatings on the cutting tools involved are mainly nitrides, starting from binary nitrides (TiN, CrN, etc.) to ternary nitrides (TiCN, TiAlN, etc.), up to multi-component nitrides (TiSiN, TiAlSiN, TiAlCrYN, etc.) and multilayered nitride systems (TiN/NbN, TiAlN/VN, TiAlN/CrN, etc.). The coating microstructure also evolved from columnar micrometer coatings to nanocrystalline coatings, nanolayer/superlattice coatings and nanocomposite coatings.
The development of PVD coatings followed the steps: 1st generation (1970): pseudo ceramic materials based on binary compounds (TiN, TiC, TiB2, etc.); 2nd generation (1985): ternary and quaternary interstitial solid solutions (Ti—Al—N, Ti—Al—N—C, etc.); 3rd generation (1990): multilayer structures (M/MN/M and MN/MC/MN, etc., where M—metallic component); 4th generation (up to date): nanolaminated coatings (TiN/NbN, TiN/WN, TiAlN/CrN, TiN/AlN, etc.) and nanocomposite coatings (Me-Si—N, Me=Ti, Cr, TiAl, CrAl, etc.). The 1st-3rd generation coatings are already commercialized. The 4th generation coatings has been investigated in some institutes/universities and coating companies. These coatings can be tailored to have high hot hardness, low friction coefficient, and good toughness, etc. Some of them have been applied on cutting tools. For example, nanocomposite coatings (e.g. nc-TiAlN/a-Si3N4, nc-AlCrN/a-Si3N4, nc-AlTiCrN/a-Si3N4) have been applied on high-speed steel tools and cemented carbide tools to obtain much higher tool life than the conventional TiN, TiAlN coated cutting tools. Efforts have been conducted to coat nanocomposite on ceramic cutting tools such as oxide and nitride ceramic tools by PVD techniques. It seems that the nanocomposite coating as a hard interlayer of a multilayer Al2O3-containing coating by PVD method has not been related.
One of the PVD methods to produce insulating films such as Al2O3 is radio frequency (RF) sputtering. This method can produce highly uniform films with good adherence to the substrate. However, extremely low deposition rate caused by the small DC-voltage which is generated at the target and ever growing technique problems for increasing substrate sizes have limited the use of these layers to a few applications because of the high coating costs.
Another way, the DC sputtering, which is used to grow oxides, nitrides and carbides due to the simplicity of the process, forms aluminum in a reactive argon-oxygen gas mixture. But this method cannot be performed as a stable long-term mode. Insulating layers are deposited not only on the substrate but equally on the chamber walls and on regions of the sputter targets. Here electric charges will occur that, in turn, result in arcing and therefore in a damage of the layers. Moreover, the anode function of the chamber wall or special electrodes will be impaired by insulating layers within a very short time so that the discharge extinguishes. If the target is insulated or parts of its surface becomes insulating as a result of the target poisoning by reactive gases, the charging up of Ar+ ions on the target surface takes place. This prevents the later incoming Ar+ ions from reaching the target so that the sputtering process cannot carry on further. As a result, no more secondary electrons are ejected from the target so that the glow discharge plasma cannot maintain itself and the deposition process stops. In addition, the accumulation of the Ar+ ions on insulating spots usually causes violent arcs which lead to damage of the power supply.
The utilization of bipolar pulsed power on the cathode enables deposition of dielectric films at a fairly high growth rate without target poisoning and arcing. And the deposition rate can be enhanced as compared with RF sputtering. By utilizing the bipolar pulsed power (i.e., alternatively applying a positive and negative voltage), the surface is continuously discharged. The effect of the positive pulse is to draw electrons in the plasma toward the target where they can neutralize the accumulated Ar+ ions. If the pulse duration (pulse width) and the height of the positive pulse are just enough to produce an electron current that can discharge all the accumulated Ar+ ions on the target, the repelling effect on the incoming Ar+ ions when the target voltage turns negative is eliminated. This keeps the sputtering process going and the discharge plasma can be self-maintaining. Furthermore, application of the positive pulse can lead to preferential sputtering of the poisoned spots on the target. This helps to eliminate target poisoning and arcing. Therefore, reactive magnetron sputtering deposition of oxide compounds or sputtering deposition of oxide compounds such as Al2O3, ZrO2, TiO2, and ZnO, is enabled by the introduction of bipolar pulsed-dc power on the sputtering cathode.
With the invention and development of the bipolar pulsed DMS (Dual Magnetron Sputtering) technique, it becomes possible to produce crystalline insulating layers such as Al2O3 by PVD methods. In the bipolar dual magnetron system, two magnetrons alternately act as anode and cathode and, hence, preserve a metallic anode over long process times. Bipolar pulsed dual magnetron sputtering enables high power input into the plasma during the pulsed, thus creating dense plasma with highly energetic particles. When the frequencies are high enough, possible electron charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of “arcing” will be limited. The bipolar pulsed DMS technique provides wide opportunities for depositing and producing high-quality, well-adherent, α-Al2O3 thin hard films on cutting tools. In addition, the technique involves stable process conditions within a large process window as compared to other sputtering techniques, as well as high deposition rate for insulating coatings such as Al2O3 according to U.S. Pat. No. 6,673,430 B2 and U.S. Pat. No. 6,423,403 B2, a γ-Al2O3 layer with grain size less than 0.1 μm and free of cracks and halogen impurities is deposited utilizing a DMS technique at substrate temperatures in the range 450° C. to 700° C., preferably at 550° C. to 650° C.
Another possible PVD method for depositing Al2O3 coatings on cutting tools is by the present invention of dual magnetron sputtering metal-doped conductive Al2O3 ceramic targets in an Ar gas or Ar+O2 atmosphere. In this technique, the nonconductive Al2O3 ceramic is doped by metals such as Al, Ti, Cr, Nb, Pt or conductive ceramics such as SiC. The conductive doped Al2O3 ceramic target is used as cathode applied with an asymmetrical bipolar pulsed power. Both of these two techniques had not been utilized previously for depositing Al2O3 coating layers on Si3N4-based cutting tools.